A typical cellular communication system includes one or more base stations (BS) and multiple mobile stations, as shown in FIG. 1A. Each BS defines a cell of coverage, where each mobile station (MS) can communicate with a BS via a BS-MS link while within communication range of the BS cell coverage. In many cellular systems, radio resource management (RRM) for orthogonal frequency division multiple access (OFDMA)-based cellular systems is utilized. Such systems address resource allocations (e.g., frequency, time, power), among BS-MS links (i.e., transmission channels defined by frequency carriers, spreading codes or time slots).
There are two types of RRMs: intra-cell RRM and inter-cell RRM. The intra-cell RRM tries to assign resources to MSs or BS-MS links within a cell (and prevent interference among MSs). The inter-cell RRM tries to assign resources to multiple cells (and prevent interference among BSs and MSs in different cells). Available frequency bandwidth is divided into frequency carriers and assigned to the BS-MS links based on channel conditions and traffic demands. End-to-end throughput between a BS and MS is a function of Signal-to-Interference-and Noise-Ratio (SINR) of the link between the BS and MS.
Intermediate relay stations (RS) have been used for Page: 3 improving throughput, coverage, and spectrum efficiency of cellular systems. FIG. 1B shows an example cellular system including a base station BS and multiple mobile stations (MS) and relay stations (RS1, . . . , RS6). A two-hop transmission takes place between a BS and MS via a RS, wherein a RS may be an MS itself. The introduction of relay stations brings forth new challenges to the RRM design. RRM for relay enhanced cellular (REC) systems has to address resource allocations among BS-MS, BS-RS, and RS-MS communication links.
In order to support relay stations, communication time frames can be divided into access zones and relay zones. In access zones, relay stations communicate with two-hop mobile stations. In relay zones, relay stations communicate with the BS. The BS can communicate with direct mobile stations (or one-hop mobile stations) in both access zones and relay zones. Note that access zones and relay zones are defined in time domain.
In an REC system, there are two types of cells: BS cells and relay cells. The BS cell is usually adjacent to every relay cell; therefore, it cannot share the same frequency resource with the relay cells. Nevertheless, relay cells that are well separated from each other may reuse subcarriers in their access links. Resource reuse introduces new challenges to subcarrier allocation, however. One conventional subcarrier allocation approach proposes an equal power multi-cell resource allocation process, wherein subcarriers are assigned one-by-one to one or more cells. For each subcarrier, all cells are examined, starting from the most under-assigned one and ending to the most over-assigned one. In each cell, every MS is evaluated on its contribution/harm to the overall throughput. If none of the mobile stations (MSs) in a cell can improve the overall throughput, the cell is skipped. If one or more MSs in a cell can improve the overall throughput, an MS that provides the most benefit to the throughput is assigned the subcarrier. Another conventional subcarrier allocation approach proposes a multi-cell resource allocation process which takes into account minimum data rate constraints. For each subcarrier, all under-assigned MSs are evaluated, starting from the MS that provides the most benefit to the overall throughput. MSs are added to share the subcarrier until no more MSs can benefit the overall throughput. Once a MS achieves its desired data rate, it shall not accept more subcarriers. This constraint is removed when all MSs meet their data rate requirements.
However, such subcarrier allocation approaches are not applicable to subcarrier allocation in REC systems that allow resource reuse among relay cells. Such conventional multi-cell resource allocation approaches handle all cells in the same way. As noted, in an REC system, there are two types of cells: BS cells and relay cells. Because resource reuse is allowed only among relay cells, the relay cells and the BS cell shall be handled differently. Therefore, the conventional approaches cannot be applied.
Another conventional approach proposes a three-step subcarrier allocation process for REC systems without resource reuse. First, subcarriers are allocated independently in the access zone and the relay zone to maximize the throughput of each zone. Specifically, each subcarrier is assigned to the communication link (including BS-MS, RS-MS, and BS-RS links) that can achieve the highest data rate increase with the subcarrier. And then, subcarriers are reallocated from over-balanced cells/links to under-balanced cells/links in both the access zone and the relay zone until no improvement could be achieved. Lastly, the power levels of relay links and access links are adjusted to balance capacities of the two-hop links. However, such a three-step approach is directed to REC systems which do not allow resource reuse (FIG. 2). In other words, the base station cell does not share the same frequency resource with the relay station cells. The relay station cells do not share the same frequency resources among themselves, either. When resource reuse is allowed in an REC system, conventional resource allocation approaches are not suitable for subcarrier resource allocation. This is because in a REC system that allows resource reuse, a subcarrier may be assigned to access links in two or more relay cells. Once the subcarriers are assigned, it is impractical to reallocate subcarriers among relay cells since each reallocation (adjustment) affects not only the cells releasing/accepting the subcarrier, but also all other cells that are assigned the same subcarrier.